1. Field of the Invention
The present invention relates to a Zoom System for imaging a laser beam onto a target plane.
2. Description of Prior Art
A laser slit lamp for photocoagulation is offered, for example, under the designation "VISULAS Argon II", and is described, for example, in the Carl Zeiss brochure with the printer's imprint 30-374-D (W-TS-I/92 T 00). In this system, the laser beam is conducted via a glass fiber to the slit lamp of the slit lamp microscope, and is coupled via a subsequent optical system into the illuminating beam path. The optical system subsequent to the glass fiber images the exit face of the glass fiber onto the retina to be treated. In this case the light bundle exiting the fiber end contains both the therapy beam, for example that of an argon laser, and also a weak, visible targeting beam, for example that of a laser diode.
The optical system, for imaging the exit face of the glass fiber, firstly contains a collimator which collimates the divergent beam bundle leaving the glass fiber. For this purpose the exit face of the glass fiber is arranged in the focal plane of the collimator. The collimated beam bundle is expanded by a succeeding afocal Galilean pancratic lens with a variable telescopic magnification, and is finally focused by an objective, at the focal point of the objective. Consequently an image of the exit face of the glass fiber results at the focal point of the objective. The diameter of this image is, according to the adjustment of the Galilean pancratic lens, between 50 .mu.m and 500 .mu.m. Here, the exit face of the glass fiber has a diameter of 50 .mu.m, that is, the fiber end is imaged with a magnification between 1.times. and 10.times.. The particular range of magnification in which the exit face of the fiber is sharply imaged on the retina is termed the "parfocal" range.
A further enlargement of the spot diameter on the retina can be obtained in the so-called defocused mode, in which the image of the fiber end is defocused by an intentional offset adjustment of the afocal Galilean pancratic lens, that is, the image of the exit face of the fiber lies behind the retina. However, this defocused mode, in contrast to the parfocal range, an intensity distribution corresponding to a Gaussian bell curve is obtained, instead of a rectangular intensity profile in the parfocal range. Such a Gaussian intensity profile is unfavorable, however, for clinical application. Apart from this, this defocused mode is concommitant with a very long and very slender beam waist of the laser beam, so that beam intensities arise in the region of the cornea that under unfavorable circumstances, and with simultaneous non-compliance with predetermined limits to settings of the laser power, can lead to corneal damage. The energy of the laser beam is concentrated in a smaller cross sectional area in the region of the slender beam waist thereby increasing the energy density, i.e., intensity. Because this beam waist (having increased energy density) is long it probably includes the cornea. But because the cornea must not be exposed to high energy densities the long and slender beam waist of the defocused mode and therefore the defocused mode is potentially harmful to the cornea.
An optical system for a laser slit lamp is known from U.S. Pat. No. 5,336,216, in which a real intermediate image of the fiber end is produced by a short-focus, two-component system. The size of the intermediate image is variable by changing the distance between the two short focus components. However, an axial displacement of the intermediate image accompanies the variation of the size of the intermediate image. The real intermediate image is imaged at a 1:1 scale on the retina by means of a subsequent long focus system of two components, of which the first is coupled to the movement of the short focus system.
The result achieved by this system is that, at all spot diameters between 50 .mu.m and 1,200 .mu.m on the retina, the beam has a large cross section, and thus a low intensity, in the plane of the cornea. However, this optical system, at spot diameters between 500 .mu.m and 1,200 .mu.m, must be operated in the so-called defocused mode, that is, for spot diameters of this size the exit face of the glass fiber has to be imaged behind the retina. A further disadvantage of this known solution is its very considerable length, which excludes a compact integration of the optical system into a slit lamp. Moreover, the required displacement paths of the zoom system are very large, so that it is mechanically expensive to make. A main problem in this solution is, furthermore, that one of the short focus components is brought, in the defocused mode, into the plane of the intermediate image of the fiber end, which can result in problems because of the high power densities at this point.
Furthermore, zoom objectives for cameras are known, for example, from U.S. Pat. No. 4,576,445, which have two lens groups with overall positive converging power, and two subsequent lens groups with negative converging power. In them, the components with negative converging power are movable to vary the magnification. However, such photographic objectives are not suitable for use in laser slit lamps, because of their optical construction and because of the usually only small zoom factor of 3 to 4.